A study of growth and several other parameters affected by 17-beta-estradiol administration in estrogen (E)-sensitive pituitary and mammary tumor cells in rats of different ages and sexes is proposed. These cells which carry E receptors and grow faster in adult females than in adult males show a similar character when injected into either male or female newborn (NB) rats exclusively. A significant growth delay is seen, however, when these same cells are injected into the NB rats of both sexes. This delay seems to be mediated by the action of alpha-fetoprotein (AFP). No such delay in growth is observed when injected cells are E receptor-carrying cells but autonomous in growth (they grow at the same speed regardless of whether they are injected into males or into females). The further characterization of this delayed period of growth will be done using rat pituitary and rat mammary tumor cell culture systems that are a) E sensitive or b) autonomous. AFP will be purified by biochemical methods (gel electrophoresis, chromatography) and its effect on E target cells will be tested in the cell culture system. E and thyroid hormones will be tested for their ability to affect the growth of these target cells under a variety of conditions in animals and in cell culture. A mechanism similar to that probably responsible for the inhibition of growth of E-sensitive tumor cells should be active as well in the development of normal E2 sensitive target cells such as vagina and uterus cells. This inference is drawn based on the results of experiments described. The study of the mechanism of sex steroid action in adult rats is also proposed.